1. Field of Invention
This invention relates to a controller for an ink-jet apparatus and, more particularly, to a controller for a piezoelectric type ink-jet apparatus.
2. Description of Related Art
Ink-jet type recording devices are well known in the prior art, and typically used for recording image data outputted from personal computers, facsimile machines, and the like. This type of recording device is superior to other types of recording devices in that it is quiet and capable of recording on sheets of various materials.
FIG. 1 is an exploded perspective view of part of an ink-jet head. Illustrated is the basic construction of an ink-jet head used for a piezoelectric type ink-jet printer. The ink-jet head is formed by stacking a cavity plate 10, a piezoelectric actuator 20, and a flexible flat cable 30 in this order from the bottom. The ink-jet head is provided with cavities 16, a supply hole 19, for supplying ink to the ink-jet head, and surface electrodes 26, 27 electrically connected to piezoelectric elements 50, which will be described later. The cavity plate 10 is formed by stacking five plates.
FIGS. 2A-2C and 3A-3C are vertical cross-sectional views of the ink-jet head taken along a direction perpendicular to its longitudinal direction when the cavity plate 10 and the piezoelectric actuator 20 are stacked upside down relative to the state shown in FIG. 1. As shown in FIG. 2A, the cavity plate 10 is formed by stacking five plates, namely, a nozzle plate 34, a first plate 36a, a second plate 36b, a third plate 36c, and a fourth plate 36d. A manifold 44, a restrictor orifice 46, a cavity 16, and a communication passage 48 are formed in corresponding plates 36a-36d. A nozzle 32 is formed in the nozzle plate 34 and ink in the communication passage 48 is ejected therethrough. The manifold 44 communicates with the supply hole 19 through a passage (not shown). In the ink-jet head, 75 sets of cavities 16 and nozzles 32 are arrayed in a row and another 75 sets of cavities and nozzles, which are bilaterally symmetrical with those shown in FIGS. 2A-2C, are arrayed in a row. A total of 150 sets of cavities and nozzles are arrayed in two rows such that 150 nozzles are aligned in a row. The piezoelectric actuator 20 is provided with a plurality of piezoelectric elements 50, which are placed adjacent to the cavities 16.
In a state shown in FIG. 2B, a voltage is applied to the piezoelectric element 50 to expand the piezoelectric element 50. When the application of a voltage to the piezoelectric element 50 is stopped, the piezoelectric element 50 contracts, as shown in FIG. 2C, and a negative pressure is developed in the cavity 16. Then, ink flows from the manifold 44 to the cavity 16. Upon reapplication of a voltage to the piezoelectric element 50, it expands again, as shown in FIG. 3A, and the ink that has flowed in is pressurized and ejected as a main ink droplet I from the nozzle 32. The above-described operation is repeated a specified number of times, according to a drive waveform supplied from a control circuit to the ink-jet head, to form a dot having the desired density. In short, a plurality of drive pulses are supplied to the ink-jet head in order to form a dot having the desired density.
When two drive pulses are supplied, the second pulse is supplied with such timing as to increase the residual pressure wave vibration in the cavity 16 generated by the first pulse. As a result, the second ink droplet is efficiently ejected.
In this case, however, an extra droplet called a satellite droplet S may be generated in addition to the main ink droplet I, as shown in FIG. 3B. This may occur when a plurality of droplets are continuously ejected to form a dot. If the pressure wave vibration in the cavity 16 is not reduced sufficiently after the main droplet I has been ejected, such residual pressure wave vibration will cause ejection of extra ink in the form of a satellite droplet. If this occurs, a finished printout may be undesirably altered. This may be especially so if a satellite droplet is ejected when no dot is formed next to the currently formed dot while using the same nozzle 22. In this event the satellite droplet can be seriously noticeable. Even if such a satellite droplet is not formed, formation of the next dot may become unstable due to the pressure wave vibration. To prevent generation of such an extra ink droplet, a cancel pulse (stabilizing pulse) is conventionally added. For example, when two pulses are supplied as described above, a cancel pulse is supplied following the second drive pulse with such timing as to cancel the residual pressure wave vibration in the cavity 16. In another conventional method, a first cancel pulse is supplied following the first drive pulse to cancel the residual pressure wave vibration, and a second cancel pulse is also supplied following the second drive pulse.
FIG. 4 shows a timing chart showing generation of a drive waveform having a cancel pulse. Upon generation of a strobe signal that regulates operation of the ink-jet head, dot data including the dot density is inputted to the control circuit of the ink-jet head. Then, the control circuit determines a drive waveform based on the received dot data and clock signals that regulate pulse generation.
A cancel pulse is especially important when no ink is ejected at a print cycle for the next dot. More specifically, when ink is ejected at a print cycle for the next dot, the next ink ejection will be less affected by the residual pressure wave vibration even if it is not attenuated sufficiently. However, when no ink is ejected at a print cycle for the next dot, the above-described satellite droplet will be generated by the residual pressure wave vibration, if it is not attenuated sufficiently.
Whether ink is ejected at each print cycle is determined based on the dot data stored in an image memory.
When the control circuit determines that the current dot data indicates ink ejection and the next dot data indicates no ink ejection, the control circuit selects a drive waveform having a cancel pulse CP to form the current dot. When the piezoelectric element 50 is driven according to the drive waveform having a cancel pulse CP, the pressure wave vibration in the cavity 16 is stabilized, thereby preventing generation of a satellite droplet S or unstable ink ejection, as shown in FIG. 3C. Although, in FIG. 4, a cancel pulse PC is inserted at the end of a drive waveform, it may be inserted in the middle of a drive waveform, or a plurality of cancel pulses may be inserted within a single drive waveform. In the above-described techniques, however, the length of a drive waveform is elongated because a cancel pulse is inserted into an original drive waveform required just for forming a dot. Setting the print cycle based on the elongated drive waveform will reduce the operating speed of the ink-jet head.
Another problem with the case where a plurality of drive pulses are supplied to the ink-jet head to form a dot is that when ink is ejected continuously over two print cycles to form two dots, the time interval between the last drive pulse for the first dot and the first drive pulse for the second dot may become short, depending on the number of drive pulses. As a result, the residual pressure wave vibration in the cavity may not be attenuated in such a short time interval, resulting in unstable ink ejection for the second dot.
It is an object of this invention to provide an improved controller for an ink-jet apparatus that can perform high-speed printing and can perform stable ink ejection when ink is ejected continuously over two print cycles.
One aspect of the invention involves a controller for an ink-jet apparatus. The controller includes an ink-jet head that ejects ink from a cavity and a waveform generator that generates a plurality of waveform signals. The waveform signals are issued at predetermined print cycles to the ink-jet head, which forms dots sequentially, according to the plurality of waveform signals, on a print medium while moving relative to the print medium. A waveform selector selects one of the plurality of waveform signals based on whether dot data indicates ink ejection for the two adjacent print cycles. The waveform selector then outputs a selected waveform signal to the ink-jet head.
The waveform generator generates a plurality of waveform signals including a waveform signal extending over two adjacent print cycles. The waveform selector selects the waveform signal extending over two adjacent print cycles when the dot data for a current print cycle indicates ink ejection and the dot data for a next print cycle indicates no ink ejection.
Accordingly, when a dot is formed by ink ejection at the current print cycle, followed by no ink ejection at the next print cycle, the controller generates a waveform signal extending over two adjacent print cycles. Thus, high-speed printing can be achieved without elongating the print cycle.
According to another aspect of the invention, an ink-jet apparatus sequentially forms dots on a print medium by moving relative to the print medium and includes: a cavity plate having a cavity from which an ink droplet is ejected; an actuator that changes the pressure in the cavity; and a controller that outputs drive pulses, at predetermined print cycles, to the actuator based on dot data. When dot data for a current print cycle indicates ink ejection, while dot data for a next print cycle indicates no ink ejection, the controller continuously outputs a plurality of drive pulses to the actuator to cause ejection of a plurality of ink droplets from the cavity to form a dot. This occurs after a certain delay from a start of the current print cycle.
Accordingly, when a plurality of drive pulses have been continuously outputted at the previous print cycle, the time interval between the last drive pulse at the previous print cycle and the first drive pulse to be outputted at the current print cycle becomes longer than that obtained under conventional control. During such a long interval, the residual pressure wave in the cavity generated by the drive pulses outputted at the previous print cycle can be reliably attenuated, and ink ejection can be stably performed by drive pulses outputted at the current print cycle.